Electromagnetic acoustic transducers (EMATs) are electrical devices that can transmit and receive sound waves in electrically conducting materials without requiring contact with the material. Since sound waves reflect from defects such as cracks and voids, EMATs are typically used as inspection devices. The characteristics of the sound waves transmitted from and received by EMATs, including frequency, intensity, mode and beam shape are determined primarily by the EMAT design and electrical excitation of the EMAT components.
EMATs offer several advantages when compared to piezoelectric transducers. EMATs do not require any fluid coupling, unlike piezoelectric transducers in which the sound is produced in the probe and transferred to the material through a coupling medium such as oil or water. EMATs can inspect at greater speeds and therefore provide greater throughput when they are used in automated inspection systems. Since EMATs generate sound waves immediately below the surface of the material being tested, they provide greater accuracy, reliability and repeatability for applications in which the material is contaminated, rough, heated to elevated temperatures or moving at high speeds. Since fabrication of EMATs can be very precise, the EMAT or its components can be interchanged with minimal variation in characteristics or performance. The simple construction of EMATs provides a nearly unlimited variety of designs to facilitate shaping, steering and focusing beams to achieve the desired acoustic effects.
EMATs are typically composed of two fundamental components: magnets and coils of insulated electrical conductors. Either permanent magnets or electromagnets (magnets) are used to produce magnetic fields that penetrate the surface of the material component being tested. Coils composed of electrical conductors, commonly referred to as RF coils, are placed between the magnets and the test material. These RF coils are used to induce high frequency magnetic fields in the test material. Interaction between the fields from the magnet and the fields from the RF coils produce forces within the atomic or molecular lattice of the test material. The forces vary in intensity and direction with time at frequencies equal to those of the current in the RF coils. The oscillating forces produce acoustic or sound waves that normally propagate within the test material and away from the EMAT in two opposing directions.
Illustrated in FIG. 1 is an EMAT configuration that is used to generate vertically polarized shear (SV) waves, Lamb waves and surface waves, which are also referred to as Raleigh waves. A magnet 1 produces a magnetic field 2 perpendicular to the metal part under test, or the test material 3. A meander radio frequency (RF) coil 4 illustrated by but not limited to a meander coil composed of insulated electrical conductors is energized by an alternating power source 5, and results in alternating current 6 which flows in the RF coil 4 between its terminals. The alternating current 6 produces alternating fields 7, which encircle the eddy currents 8 and penetrate the surface of the test material 3. The penetrating alternating fields 7 induce alternating eddy currents 8 in and near the surface of the test material 3. Alternating magnetic fields 9, which encircle the eddy currents 8, are also generated in the test material 3. The alternating fields 7 from the eddy currents 8 interact with the alternating magnetic fields 9 from the magnet 1 to produce Lorentz forces 10, in the test material 3 and under each RF coil 4. These Lorentz forces 10 result in sound waves, such as horizontally polarized shear waves, which are ultrasonic acoustic or sound waves commonly known in the art as SH waves 11, which propagate from the EMAT in opposite directions in the test material 3.
Illustrated in FIG. 2 is an EMAT which uses a magnet array 12 such as an array of permanent magnets and an encircling RF coil 4 to generate SH waves 11. Part of the RF coil 4 is under the magnet array 12, and also in close proximity to the test material 3. When an alternating power source 5 is applied to the RF coil 4, eddy currents 8 and the associated alternating magnetic fields 9 are induced in the test material 3. Interaction of the magnetic fields 2 from the magnet array 12 and the alternating fields 7 from the eddy currents 8 produce Lorentz forces 10 in the test material 3, which are near the surface and also parallel to the surface of the test material 3. These Lorentz forces 10 result in SH waves 11 that propagate in opposite directions in the test material 3.
Illustrated in FIG. 3 is an EMAT, which uses a magnet 1 such as an electromagnet and RF coils 4 to produce SH waves 11 in some ferromagnetic materials 14 that exhibit the property of magnetostriction. A magnet coil 13 composed of insulated, electrical conductors is wound around a core of ferromagnetic material 14. When the magnet coil 13 is excited by electrical power source 15, a transient current 16 flows between the terminals of the magnet coil 13. The transient current 16 in turn generates a tangential magnetic field 17, a part of which penetrates the surface of the test material 3. The tangential magnetic field 17 induces transient eddy currents 18, which flow under and around the poles of the magnet 1.
RF coil 4 is excited by alternating current 6 at frequencies that are greater than the component frequencies of the transient current 16 of the magnet coil 13. Alternating current 6 in the RF coil 4 induces alternating eddy currents 8 and associated magnetic fields 9 in the test material 3. When the test material 3 exhibits the physical property of magnetostriction, the vector summation of the resultant magnetic fields 9 induced by the RF coil 4 and the tangential magnetic fields 17 induced by the magnet 1, cause expansion and contraction of the test material 3. Alternating expansion and contraction of the test material results in propagation of SH waves 11 from the EMAT in two directions.